The present invention relates to a dry-type developer and a wet-type developer for use in electrophotography, which are capable of yielding improved image density, resolution, and image fixing property, without a strike-through phenomenon that developed images on a copy are visible from the back side of the copy sheet (hereinafter this phenomenon is referred to as the strike-through phenomenon).
As a developer for use in electrophotography, a toner comprising polyethylene wax as a binder agent and an organic or inorganic pigment, such as carbon black and phthalocyanine blue, which is dispersed in the binder agent, is conventionally known. The toner particles of such a toner containing polyethylene wax have the advantage over other toner particles that higher image density can be obtained. However, the toner particles have the shortcoming that the images are depressed and defaced in the course of the image transfer process in a copying machine, which degrades the image quality, in particular, image resolution.
A trial has been carried out to prevent the decrease of the sharpness of image and resolution by using an oxidized polyethylene wax instead of polyethylene wax as proposed in Japanese laid-open patent application No. 54-97157. However, when oxidized polyethylene wax is employed, there is a problem that high image density cannot be increased any further.
In a wet-type toner, low-molecular weight polyolefins are in general use. However, the melting viscosity of such low-molecular weight polyolefins is so low that a colorant such as an organic pigment and carbon black cannot be well dispersed therein when kneaded in a kneader. Therefore high image density cannot be obtained when such low-molecular weight polyolefins are employed in the toner.
Furthermore, the melt index of the low-molecular weight polyolefins is so high that the image areas developed by a toner containing such a polyolefin are fused and tend to flow like a melted candle when heated along the fibers of a copy sheet, when heated in the course of the image fixing process. The result is that the previously mentioned strike-through phenomenon so as to be totally unsuitable for duplex copying, poor image resolution and poor image fixing to such an extent that developed images can be readily erased by an eraser, and the adhesion of the fused developer to an image fixing roller are caused.
In addition to the above-mentioned polyethylene wax, oxidized polyethylene wax and low-molecular weight polyolefins, polystyrene resin, acrylic resin, epoxy resin, polyester resin, and rubber are employed as binder agents for the conventional toners. However these resins also have the same or similar shortcomings as mentioned above more or less.